1. Technical Field
This invention relates to electric are spraying of metals and, more particularly, to a plasma arc transferred to a single wire tip that is fed continuously into the plasma-arc.
2. Discussion of the Prior Art
As disclosed in earlier U.S. patents by the co-inventors herein, plasma transferred wire arc is a thermal spray process which melts a continuously advancing feedstock material (usually in the form of a metal wire or rod) by using a constricted plasma-arc to melt only the tip of the wire or rod (connected as an anodic electrode); the melted particles are then propelled to a target. The plasma is a high velocity jet of ionized gas which is desirably constricted and focused about a linear axis by passing it through a nozzle orifice downstream of a cathode electrode; the high current arc, which is struck between the cathode electrode and the anodic nozzle, is transferred to the wire tip maintained also as an anode or the high current arc can be transferred directly to the wire tip. The arc and plasma jet provides the necessary thermal energy to continuously melt the wire tip, and the plasma provides the dynamics to atomize the molten wire tip into finely divided particles and accelerates the melted particles as a stream generally along the axis of the plasma. Acceleration of the particles is assisted by use of highly compressed secondary gas, directed as a converging gas streams about the plasma-arc axis, which streams converge at a location immediately downstream of where the wire tip intersects the plasma-arc, but avoid direct impingement with the wire tip to prevent excessive cooling of the plasma-arc.
Existing plasma transferred wire arc (PTWA) torches and associated apparatus of the prior art, used to generate the plasma transferred wire arc are sensitive to instabilities in the process resulting in occasional poorly atomized particles of melted or unmelted metal rather than spraying of fine molten particles. Process instabilities can occur when one or more of the following are outside of controlled or designed ranges: secondary air flow or pressure, plasma gas pressure, wire feed rate, wire arc current and torch rotational and linear movement rate. The occurrences of such instabilities are not fully predictable.
Poorly atomized particles results from multiple issue including the accumulation of melted particles which tend to agglomerate and form globules or droplets that move back up along the wire under the influence of the fluid dynamics of the plasma jet and secondary gases. Such globules or droplets can contaminate the wire tip and/or release the globules for projection that produces a non-uniform deposit. Process instabilities that allow particles to agglomerate may have their origin in a change of electrode shape or nozzle shape over time due to wear, buildup of contaminants, or due to irregularities such as the rate of wire feed by the automatic feeding mechanism or changes in the level of current passing through the wire.